1. Field of the Invention
The present invention relates to an arc resistant insulator which is suitable for use, for instance, as an arc-extinguishing plate for a blow-out type circuit breaker, and a process for its preparation.
2. Description of the Prior Art
Various types of arc resistant insulators are used including asbestos products prepared by binding asbestos fibers with phosphoric acid or a metal phosphate and cement, or porcelain products made of cordierite, zircon or the like.
The asbestos products are superior in the arc-extinguishing performance and have high strength, particularly high impact strength by virtue of the fibrous asbestos, and they are useful as arc-extinguishing plates for e.g. unit switches for vehicles which are subject to vibrations. The arc-extinguishing performance of the asbestos products is largely dependent on the moisture (adsorption water and constitution water) contained in the asbestos, and the moisture acts effectively against arcs at the initial stage by diffusing and cooling the arcs, whereby good results are obtainable with a short breaking time. However, when subjected to arcs repeatedly, the asbestos products will lose the moisture, and at the same time, the fiber structures of the asbestos will be destroyed to form glass and will undergo undesirable deformation which is so-called shrinkage.
Further, the asbestos products are generally inferior in their electrical insulating performance in a high humidity atmosphere and accordingly they are restricted in the range of applications as insulators.
On the other hand, the porcelain products are made of material having a high melting point. When subjected to arcs, they form a glassy texture, but do not undergo substantial deformation as contrasted to the asbestos products. Thus, they are good arc resistant insulators. However, the porcelain products are inferior in their impact strength, and accordingly they are not very much suitable for use under high vibration conditions, e.g. for use in vehicles. Besides, they present a difficulty in the dimensional precision.